Current System AnalysisPlanetary Mission Analysis Using Microspacecraft Technology Systems

Tony Statom

November 2, 1992

IntroductionThe Jet Propulsion Laboratory using the developments in microtechnology from DARPA and SDIO are investigating the potential use of this technology in Microspacecraft.

The history of spacecraft shows that from the small mass spacecraft that the space program began with the mass has steadily increased since then. The increased mass of spacecraft are causing mission frequency to decrease. This is the driver for the development of spacecraft using microtechnology. The decreased mass should increase launch frequency.

Definitions for spacecraft are as follows[3]:

Standard

1000 kg

Small

100 kg

Microspacecraft

10 kg

Microspacecraft Workshop Summarization
In July of 1988 NASA and SDIO sponsored a workshop at JPL titled "Microspacecraft for Space Science". The results are[2]:

Microspacecraft (1-10 kg) are technically feasible.

There is a class of scientific and exploration missions that can be enabled by microspacecraft. This class of missions requires many simultaneous measurements displaced in position, as on the surface of a planet or small body or in the region of space. The enabling feature of microspacecraft is the assertion that using many microspacecraft (1-10 kg) will cost less (spacecraft and launch costs) and involve less risk than using large (500-1000 kg) spacecraft for such missions.

Other missions enabled by the microspacecraft concept are those that require very high mission delta-V's.

While useful and perhaps enabling for the types of missions mentioned above, microspacecraft are not applicable to all types of space exploration and science missions and should not be viewed as a panacea.

Microspacecraft TechnologyThe SDIO and DARPA programs have yielded developments that are being used in conceptual designs including most spacecraft subsystems.

POWERSolar arrays or nuclear (radioisotope) sources can be used to generate power. A 1 x 2 meter inflated solar array that can be stored in a 10.2 cm diameter and 1 m long canister[2]. The beginning life power is 125 watts and 3 year end of life power of 100 watts with a mass of 0.66 kg[2]. A microradioisotope thermal electric generator (RTG) with a mass of 159 grams produces 2 watts at 5 volts after 5 years[2].

PROPULSIONOne of the objectives from the SDIO investigation was small projectiles with excellent maneuverability. A small propulsion subsystem with a mass of around 50 grams each using standard bi-propellants can be used for main engines and 4 small engines for attitude control thrusters with a mass of less than 3 kg an rated over 600 N each[2]. SDIO also contributed to the development of a small upper stage called the Advanced Liquid Axial stage (ALAS). Specific impulse about 345 seconds. Wet mass of around 7.7 kg and dry of about 1.8 kg[2].

POSITION DETERMINATIONA small unit developed by the SDIO program was a system with mass of 150 grams called Quartz Rate Sensor (QRS). The inertial rates are measured by the quartz tuning fork elements, and linear accelerations by the silicon accelerometers. This system was made by Systron Donner[2]. It draws 7 watts of peak power.

The Lawerence Livermore National Laboratory has developed a miniature star tracker camera the prototypes mass is about 300 grams[2].

The developments in Scanning Tunnelling Microscopy (STM) technology allows the use of the quantum-mechanical electron tunnelling to be used for position detection. This sensor would be small in size. The sensor is a few Angstroms[5]. This sensor can also be used for a Martian microseismometer and microweather station[2].

DATA PROCESSORA parallel processor using two dimensional hybrid wafer scale integration has a mass of about 3.6 kg is being developed by The Space Computer Corp. This processor will have 1.3 giga floating point operations per second. A future version using three dimensional hybrid wafer scale integration is expected to have a mass of about 0.8 kg[2].

TELECOMMUNICATIONSA conceptual design using optical frequencies has been done. It uses a semiconductor laser at a wavelength of 8.0 x 10-7 m. The subsystem uses about 6 watts of power, a 10 cm aperture, and should weigh about 1 kg. Performance at night is 1000 bits per second from 1 AU to a 10 m diameter receiver located Earth with clear skies. With daytime background noise the performance drops to 1 bit per second[2].

CAMERAA conceptual design for a camera has a mass of less than a kilogram uses 4 watts in operation and has a resolution of 7 meters per pixel at a range of 100 km. The design assumed a spinning rate of 5 rpm or less. The system included a data buffering which caused a read out rate to the microspacecraft of about 10 bits per second[2,6].

LAUNCH VEHICLEUsing microspacecraft gives greater options in the choice of a launch vehicle. With these small systems greater delta V can be obtained. A microspacecraft launched at 50 km/s would reach 10 ,100 ,1000 AU from the Sun in 0.6, 6.2 and 62.8 years respectively. This velocity might be obtained in the future by electromagnetic launchers[4]. If conventional chemical launch vehicles are used and a microspacecraft is launched at 10 km/s then 10, 100 and 1000 AU from the Sun can be reached in 2.2, 34 and 390 years respectively[4]. This assumes:

Impulsive delta V of between 10 and 50 km/s.

The payload is launched from a 500 km circular Earth orbit.

The payload is launched at the right time and in the right direction in order to take advantage of its orbital energy relative to both the Earth and the Sun.

The payload is launched on a heliocentric, hyperbolic escape trajectory[4].

Missions/Systems DescriptionsAt JPL the three areas of focus in the current microspacecraft effort are:

Bringing the Asteroid Investigation with Microspacecraft (AIM) concept to the next level of feasibility by backing off on some technology assumptions, developing more detail it the spacecraft design, building a full-scale mockup of the spacecraft, and holding technical peer reviews.

New conceptual studies to increase the number of possible missions considered for microspacecraft[1].

Astroid Investigation With Microspacecraft (AIM)The mission objective is to flyby three separate near Earth asteroids and return high resolution images[1]. Constraints:

Sun range between 0.8-1.2 AU

Earth-S/C range<1.6 AU

Post launch delta V <200m/s [1].

Some subsystem descriptions:

Launch vehicle - Pegasus carrying three microspacecraft[1].

Telecommunications - can achieve 100 bps at 1.6 AU to the 34 m HEF ground stations of the DSN[1].

Summary and ConclusionWith the advances in microtechnology applicable to spacecraft through the developments of DARPA and SDIO microscpacecraft conceptual designs are being analyzed. The microspacecraft gives greater flexibility in the choice of a launch vehicle. This flexibility should drive down mission costs and increase launch frequency. While there are many missions that can utilize microspacecraft it is not viewed as all inclusive.

The microspacecraft analysis is not complete. Greater fidelity to the analysis both technically and economically must be done. Subsystem component options should be narrowed. Power sources with long duration must be developed to return data over interplanetary distances.

This type of approach to spacecraft system will continue as microtechnology makes greater strides in research. As microtechnology continues to decrease in size and expand in ability it should be considered in initial conceptual design to assess feasibility.

AcknowledgementRoss Jones the supervisor of the Advanced Spacecraft System Concepts Group at JPL who sent me three of his papers that this system analysis was based on.